The kinetics and erythropoietin requirements of morphologically unrecognizable erythroid progenitor cells (BFU-E, CFU-E) will be studied by in vitro culture assays from the following viewpoints: 1) chronic inflammation induced in mice by turpentine abscess; 2) after severe suppression of committed progenitors by BCNU injection; and 3) after severe suppression of pluripotential stem cells (CFU-S) and of erythroid progenitors by Myleran.